1. Field of the Invention
The present invention relates generally to electromagnetic-based well logging systems. The present invention relates more specifically to a method and apparatus for making electromagnetic wave resistivity well logging tools azimuthally sensitive through the use of partially shielded antennas.
2. Description of the Prior Art
Among the various types of well logging systems and methods are those based upon the transmission, propagation, and reception of electromagnetic waves within the formation surrounding a borehole. A great deal of information can be obtained about the characteristics of the surrounding formation by directing electrical currents and electromagnetic fields into the formation and detecting changes in those currents and fields after interaction with the formation.
In instances where electrically conductive drilling mud is utilized within a borehole, a direct current measurement of the resistivity of the surrounding formation can be achieved. Alternatively, static and dynamic electromagnetic fields can be generated from a drill string into the surrounding formation, even when the drilling mud is not conductive, in a manner that permits the detection and measurement of changes in the electromagnetic field brought about by the surrounding formation. Various induction well logging techniques generate magnetic fields in the surrounding formation which are modified by the conductivity and magnetic permeability of the formation. The changes in the magnetic field brought about by the formation can then be detected by additional induction coils positioned on the drill string.
In the process of electromagnetic wave resistivity well logging, it is often desirable to be able to measure formation resistivity at different radial distances from the borehole. This process of measuring resistivity as a function of radial distance constitutes making such measurements at different "depths" of investigation. Various attempts in the past have been made to produce resistivity logs that distinguish resistivity at various radial depths from the borehole.
U.S. Pat. No. 5,389,881, issued to Bittar et al. on Feb. 14, 1995, and commonly owned by the Assignee of the present application, entitled Well Logging Method and Apparatus Involving Electromagnetic Wave Propagation Providing Variable Depth of Investigation by Combining Phase Angle and Amplitude Attenuation, discloses one such effort. This patent describes a number of improvements directed toward accurately identifying resistivity values at variable depths from the borehole using electromagnetic well logging methods. In addition, this patent describes the general state of the art and identifies various other patents that attempt to improve the quantitative measurement characteristics of electromagnetic well logging systems.
Other efforts to provide variable depth measurements include; U.S. Pat. No. 5,278,507, issued to Bartel et al. on Jan. 11, 1994, entitled Well Logging Method and Apparatus Providing Multiple Depths of Investigation Using Multiple Transmitters and Single Receiver Pairs Having Depth of Investigation Independent of Formation Resistivity.
Various other issued patents describe attempts to improve the data acquired by electromagnetic well logging systems through efforts to eliminate spurious and unwanted electromagnetic interferences. Most of these attempts involve structural efforts to shield or orient antennas in such a way that only those electromagnetic field variations of interest are transmitted and/or detected. Examples of such patents include; U.S. Pat. No. 4,536,714, issued to Clark on Aug. 20, 1985, entitled Shields for Antennas of Borehole Logging Devices; and U.S. Pat. No. 5,530,358, issued to Wisler et al. on Jun. 25, 1996, entitled Method and Apparatus for Measurement-While-Drilling Utilizing Improved Antennas. Each of these patents, and others in the same field, are directed to radial arrays of shields that effectively act as "polarizers" for the electromagnetic waves transmitted and detected by the drill string logging devices. Basically these shielding attempts establish slots around the antennas that are oriented parallel with the axis of the drill string in a manner that eliminates or reduces transverse magnetic mode noise. In this manner more specific variations in the electromagnetic field can be detected and measured.
Other patents along this same line include U.S. Pat. No. 4,785,247, issued to Meador et al. on Nov. 15, 1988, entitled Drill Stem Logging With Electromagnetic Waves and Electrostatically-Shielded and Inductively-Coupled Transmitter and Receiver Elements; U.S. Pat. No. 4,514,693, issued to Meador on Apr. 30, 1985, entitled Dielectric Well Logging System With Electrostatically-Shielded Coils; U.S. Pat. No. 4,949,045, issued to Clark et al. on Aug. 14, 1990, entitled Well Logging Apparatus Having a Cylindrical Housing With Antennas Formed in Recesses and Covered With a Waterproof Rubber Layer; U.S. Pat. No. 4,651,101, issued to Barber et al. on Mar. 17, 1987, entitled Induction Logging Sonde With Metallic Support; and U.S. Pat. No. 4,808,929, issued to Oldigs on Feb. 28, 1989, entitled Shielded Induction Sensor for Well Logging.
Still further attempts in the field are directed towards providing structural integrity to antenna coils positioned on drill strings for both wireline and measurement-while-drilling applications. Included among these are; U.S. Pat. No. 5,631,563, issued to Moriarty on May 20, 1997, entitled Resistivity Antenna Shield, Wear Band and Stabilizer Assembly for Measuring-While-Drilling Tool; U.S. Pat. No. 5,563,512, issued to Mumby on Oct. 8, 1996, entitled Well Logging Apparatus Having a Removable Sleeve for Sealing and Protecting Multiple Antenna Arrays; U.S. Pat. No. 4,511,843, issued to Thoraval on Apr. 16, 1985, entitled Electromagnetic Logging Sonde Having Improved Housing; and U.S. Pat. No. 4,940,943, issued to Bartel et al., entitled Method and Apparatus for Optimizing the Reception Pattern of the Antenna of a Propagating Electromagnetic Wave Logging Tool.
Finally, there have been a few efforts in the prior art to make electromagnetic well logging systems directional in their investigation. Heretofore, electromagnetic well logging systems have only been able to identify formation characteristics that vary with a first depth dimension (associated with a distance below the surface) and a second depth dimension (associated with a distance radially from the drill string). Heretofore, it has not been possible to distinguish formation characteristics on one side of the drill string from formation characteristics on an opposite side of the drill string at the same depth and distance into the formation. U.S. Pat. No. 4,697,190, issued to Oswald, entitled Borehole Located Directional Antennae Means for Electromagnetic Sensing Systems describes a modest attempt at creating a directional antenna for an electromagnetic sensing system for use in a borehole environment. This earlier attempt describes a V-shaped antenna with a metallic backing intended to direct the electromagnetic waves out from the axis of the drill string. The Oswald device, however, suffers both from complexity and fragility that prohibit its use and application in most borehole drilling environments. Other attempts involve complex coil positioning systems that angle the electromagnetic field transmitters and receivers to provide directional sensitivity. An example of this latter approach can be found in U.S. Pat. No. 5,508,616, issued to Sato et al. on Apr. 16, 1996, entitled Apparatus and Method for Determining Parameters of Formations Surrounding a Borehole in a Preselected Direction.
Efforts in the past to provide a directional capability to electromagnetic based well logging systems have generally been unable to provide systems capable of operation in MWD applications. Even where such systems have been developed for wireline based well logging applications, their complexity leaves them easily damaged and their inaccuracy makes their practical use questionable. In addition, efforts in the past to provide directional sensitivity to electromagnetic well logging systems have generally been unable to combine such abilities with other improvements of the type described above for radial depth measurements.
It would be desirable, therefore, to create and implement a structure and process that allows an electromagnetic wave resistivity logging tool to incorporate a directional parameter to permit the investigation of a formation in a specific radial direction within the borehole. It would be desirable for such a system to be capable of implementation in either a wireline or MWD application. It would further be desirable if such a device were structurally simple and easy to implement on conventional electromagnetic logging tools without the necessity of designing and creating an entirely new logging tool. It would be important for such a device to be versatile in its design and application so as to be useful in a variety of formations and a variety of mud conductivities.
It would further be desirable if a directional electromagnetic well logging tool could operate by both scanning all directions at a specific depth of investigation and by scanning in a single direction at a variety of depth investigations.